防風林配置對定砂效果之風洞試驗研究

Abstract

台灣西部沿海地區飛砂問題嚴重，雖防風林帶已廣泛種植以防杜風害與砂害，然防風林帶樹種組合之不同，對於定砂效果必然相異，且近年來國內廣泛推廣混交林以強化更新林相之防護機能。本研究針對低木、高木與混交林等三種林況，分別設計十組不同林型，每種林型厚度皆為4H，分別為純高木組成之A-8型、純低木組成之B-8型與林型1～8之混交林型等共十組，利用風洞設備進行吹砂試驗，探討定砂功效之異同，低木樹種選擇黃槿(Hibiscus tiliaceus L.)，高木樹種則選擇木麻黃(Casuarina equisetifolia L.)作為代表，實驗用砂取自大安溪與大甲溪出海口處飛砂嚴重區域，並配合砂粒物性分析以了解砂粒之特性。 物性分析結果：大安溪與大甲溪現地含水率皆在1.2%以下，平均粒徑為0.14mm與0.22mm，平均密度為2.72g/cm3與2.66 g/cm3，質地皆為砂土，大甲溪之砂粒較為接近球型。 風洞試驗結果：砂粒粒徑範圍0.25～0.84mm時，起動風速介於5.0～8.0m/s之間，砂粒粒徑小於0.25mm時起動風速為4.0m/s；防風林帶最佳攔砂厚度約為2.5H～3H，不同持續時間對林後飛砂量變化影響不大，空流場量測結果邊界層厚度約25cm。 B-8型（黃槿純林）林帶後方堆砂量最多，砂丘主要產生在林帶後方，定砂效果最差，故配有高木之林型定砂效果優於純低木之林型；林型4（前高後低）定砂效果高達90%，然林帶內砂丘有漫延至林帶後方之現象，若考慮堆砂緩慢，且對林帶後方防護較佳而定砂效果次之林型1（低高單排交錯）為最佳之定砂林型。

The coastal areas at the western Taiwan have serious problems of aeolian sand. Although the windbreak forest belt has been extensively planted to prevent wind damage and sand damage, however, the different sand fixation effect between difference combination of tree species of windbreak forest belt will be dissimilar inevitable. Moreover, in recent years, mixed layered forest has been widely promoted in Taiwan to enhance the protection function of renewal forest. The research aimed at three kinds of forests conditions of low woods, high woods, and mixed forest to design ten different forest types. The thickness of each forest type was 4H, and they were: Type A-8 consisted of pure high wood, Type B-8 consisted of pure low wood, and eight mixed forest types of Type 1~8. The sand blowing test in wind tunnel was utilized to explore the difference of sand fixation effect. The tree species of low wood adopted Linden hibiscus (Hibiscus tiliaceus L.), and Beef Wood (Casuarina equisetifolia L.) was chosen as the representative of the tree species of high woods. The experiment sands were got from the districts of serious aeolian sand at the mouths of Da-an River and Dajia River, and the research cooperated with the physical property analysis to understand the features of sands. The results of physical property analysis: The on-site moisture content of Da-an River and Dajia River was both under 1.2%; the average grain diameter was 0.14mm and 0.22mm; the average density was 2.72g/cm3 and 2.66 g/cm3; the texture was both sandy soil, but the sands of Dajia River was closer to ball shape. The results of wind tunnel test: The start-up wind speed was between 5.0 and 8.0m/s when the range of sand grain diameter was 0.25～0.84mm; the start-up wind speed was 4.0m/s when the sands diameter was smaller than 0.25mm; the best sand fixation thickness was 2.5H~3H; different duration did not have much influence on the variation of aeolian sand quantity behind the forest; the air flow field measuring results: the boundary layer thickness was 25cm approximately. Type B-8 of pure forest of Linden hibiscus had the most sand accumulation quantity behind the forest belt; the sand hill accumulated behind the forest primarily, and its sand fixation effect was the worst. Therefore, the sand fixation effects of forest types with high woods were better than the forest type with pure low wood. Forest Type 4’s sand fixation effect achieved 90%; however, the sand hill within the forest belt had the phenomenon of stretching to the place behind the forest belt. If considering the forest type of slow sand accumulation, better protective property to the place behind the forest belt, and second sand fixation effect, Type 1 (low wood and high wood staggered by single row) was the best sand fixation forest type.